Genetics Test Review Problems

GENETICS TEST REVIEW PROBLEMS: Write the answers on separate paper

1. In the cross Aa X Aa, how many different genotypes can occur in the gametes of the first parent? The second parent? The offspring? What is the expected phenotypic ratio of the offspring produced by this mating? The genotypic ratio? What is the probability that a given offspring of this cross will have the genotype aa? How about Aa?

2. In the cross Aa X aa, what genotypes can occur in the offspring? What is the probability that a given offspring will be Aa? If the first offspring is aa, what are the chances that the next will also be aa? What are the odds that the third will be AA? Which of the above parents is homozygous? Which displays the dominant trait?

3. In the cross AaBb X AaBb, how many different genotypes can occur in the gametes of each parent? What is the probability that a given offspring will display both dominant traits? Both recessive traits? The first dominant trait along with the second recessive trait? The first recessive trait along with the second dominant trait? How does answering these questions provide you with the expected phenotypic ratio for this cross?

4. In the cross TtSs X TtSs, what is the probability that a given offspring will have the genotype TTSS? How about Ttss? How about ttss?

5. In humans, cystic fibrosis is inherited as an autosomal recessive trait. Persons who have the genotype CC are normal both genotypically and phenotypically. Those who are Cc are phenotypically normal, but are genotypically carriers. Persons who are cc have cystic fibrosis. The parents of a child with cystic fibrosis express disbelief because no one in either family ever had the disease. Is this surprising?

6. What are the genotypes of the parents in the previous problem? What are the chances that their next child would have cystic fibrosis?

7. If a woman with cystic fibrosis marries a normal man who does not carry the recessive allele, what would be the possible genotype(s) of their children?

8. In humans, the ability to digest milk sugar (lactose) is based on an autosomal dominant allele (L). Persons who are homozygous recessive (11) cannot digest lactose once they reach adulthood. Heterozygotes are phenotypically normal. If two parents are both Ll, what are the odds that a given child, once grown, will not be able to digest lactose? If these parents have 8 children, what is the "best guess" as to how many will be able to digest lactose? Why is this answer a guess?

9. In the cross CcLl X CCLL, can a child be produced who has cystic fibrosis AND cannot digest lactose? How about in the cross CcLl X ccll? How about the cross CcLl X CcLl?

10. In humans, Rh factor in red blood cells is the result of an autosomal dominant allele (Rh). Persons who are rhrh have red blood cells lacking the factor. When a mother's red cells do not have the factor, and a fetus' red cells do, the fetus may have Rh disease. This condition is caused as maternal antibodies against the factor cross the placental barrier into the fetal bloodstream and begin to destroy red cells. If an Rh- woman (without the factor) bears an Rh+ child (with the factor), what was the father's blood type (positive or negative)? What was his genotype? With only the information given, could one predict with certainty that this women's next child would also be positive?

11. Would an Rh+ woman ever have to worry about producing a child with Rh disease? Why or why not? How about an Rh- woman with an Rh- husband?

12. A woman who is Rh- marries a man who is Rh+. The man's mother is Rh-. What is his genotype? What are the chances that this couple's first child will be Rh+? How about Rh-? How about their second child?

13. A child is born with Rh disease. What are the blood types of the child, the mother, and the father?

14. An Rh+ woman whose mother was Rh- is unable to digest lactose. She marries an Rh- man who is heterozygous Ll. What types of children can they have? 15. Huntington's disease is a rare disease of the central nervous system resulting from a dominant autosomal allele (H). The disease does not begin to express itself until about age 40. Persons who are hh are phenotypically and genotypically normal. Persons who are Hh ave the disease. Individuals who are HH simply do not occur. A couple have had 4 children and then at age 43, the father develops Huntington's chorea. What is the father's genotype? The mother's? What is a given child's chances of having the disease? What is the "best guess" as to how many of these 4 children will have the disease? Could all 4 have it? Explain.

16. Incomplete dominance (Co- dominance) is a term used to describe a condition wherein more than one allele exists for a gene, but one is not dominant over the other(s). In other words, in the heterozygous condition both alleles are expressed. In snapdragons flower color is so controlled. There are 2 alleles. One designated as RR calls for red flowers; the other designated as RW calls for white flowers. The RRRW individual has pink flowers. What kinds of offspring would one expect from a cross of a pink-flowered and white-flowered plant? A cross of 2 pinks? A cross of a red and white?

17. In humans a condition known as hypercholesterolemia is inherited on the basis of incomplete dominance. The allele CH calls for abnormally high levels of blood cholesterol; the allele CL calls for normal levels. Persons who are CHCH usually die of heart attack by age three, CHCL Individuals have moderately high levels of cholesterol, and CLCL individuals are phenotypically normal. In the case where two CHCL individuals marry, what is the probability that a given offspring will have the more serious form of the disease? The milder form of the disease? Be completely normal? Could the cross CHCL X CLCL produce a severely diseased baby? How about a mildly diseased baby?

18. Sickle cell anemia is another incompletely dominant trait. The allele CS results in the production of abnormal hemoglobin; the allele CN calls for normal hemoglobin. Individuals who are CSCN are mildly diseased. Those who are CSCS are severely diseased and usually die in childhood. Two mildly diseased persons are planning to marry. What might a genetic counselor advise this couple as to the various probabilities concerning sickle cell anemia in their children? What assumptions would you make concerning the genotypes of the four parents of this couple?

19. Some genes possess more than 2 alleles. For instance, the gene for the ABO blood groups has 3 alleles. The allele designated as IA promotes the production of antigen A on red blood cell membranes, IB the production of antigen B, and i the production of no antigen. The IA and IB alleles are incompletely dominant, and are both dominant over the i allele. This results in 6 different genotypes for this gene: Genotypes IAIA IAi IBIB IBi IAIB ii Phenotypes type A type A type B type B type AB type O

What are the chances that the cross IAIB X Iai will produce a type A baby? A type B baby? A type AB baby?

20. A type A mother with a type A husband brings home a type O baby hospital make a mistake? What if the baby had been type B? What is the mother were type O and the baby was AB?

21. Mary's mother is type O and Mary is type B. George's blood type is A. Can George and Mary have a body with type O blood? How about type A? Type B? Type AB?

22. Hemophilia is a sex-linked recessive disease having this gene located on the portion of the X chromosome that is non- homologous to the Y chromosome. Females thus possess 2 alleles for the trait while males have only 1. The allele designated h results in blood clotting failure while the H allele results in normal clotting. Females that are HH are norma, Hh individuals are carriers and hh individuals are very infrequently encountered due to the alleles extreme rareness. Males are either Hy (normal) or hy (diseased). A homozygous woman marries a man with hemophilia. Can their sons have the disease? How about their daughters?

23. A woman heterozygous for hemophilia marries a normal man. What are these person's genotypes? Can they ave male children with hemophilia? Quote the odds. How about female children with the disease?

24. In the cross HhCc X Hycc. What is the probability that the first child will be a boy with both hemophilia and cystic fibrosis? What are the odds that the first child will have both these disease?

25. A common form of color blindness is also a recessive sex-linked (X-linked) disease. Th allele C is normal while the allele c results in some degree of red-green color blindness. Can a heterozygous woman and a colorblind daughters? 26. Why are men never heterozygous for an X-linked trait? Why must men always inherit and X-linked trait from their mothers? Do daughters inherit their X-linked traits only from their fathers? Explain.

27. On the basis of allele-frequency analysis of data from a randomly mating population Snyder (1934) concluded that the ability vs. inability to taste phenylthiocarbamide (PtC) is determined by a single pair of autosomal alleles, of which T for taster is dominant to t for nontaster. Of the 3,643 individuals tested in this population, 70% were tasters and 30% were nontasters. Assume the population satisfies the conditions of Hardy-Weinberg equilibrium.

a)Calculate the frequencies of the alleles T and t and the frequencies of the genotypes TT, Tt and tt.

b) Determine the probability of a nontaster child from a taster x taster mating.

Probability (tt) child = P(Tt parent) x P(Tt parent) x P (tt child)

However, we know the parents are not tt, so we must calculate the frequency of Tt parents within the taster population (not the population at large) as follows:

= frequency Tt/ total frequency of tasters (Tt + TT)

Remember: The Chi-Square Test

An important question to answer in any genetic experiment is how can we decide if our data fits any of the Mendelian ratios we have discussed. A statistical test that can test out ratios is the Chi-Square or Goodness of Fit test.

Chi-Square Formula

Degrees of freedom (df) = n-1 where n is the number of classes